The concept of using specific laser treatment to prevent the progression of dental caries in enamel is close to becoming an exciting clinical reality. The overall objective of the studies is to develop an in-depth understanding of the interactions of laser light with dental hard tissues (enamel and dentin) and to establish a scientific basis for the safe and effective use of lasers on these tissues. The underlying hypothesis to be tested in the present proposal is that efficient conversion of light to heat results in changes in crystal composition and structure which increase the resistance of dental mineral (enamel and dentin) to dissolution by acid. Specific Aims: (1) To conduct a pilot scale clinical study of caries inhibition in living teeth in humans using caries in enamel around orthodontic brackets as a model system. (2) To test the hypothesis that laser irradiation conditions that provide caries inhibition for enamel and dentin will not have undesirable effects on the mechanical properties of the tissues. (3) To test the hypothesis that the addition of a highly absorbing layer of water to the surface of dental enamel during ablation by IR laser irradiation eliminates secondary non-apatite phases of calcium phosphate and allows the laser irradiation to markedly reduce the acid solubility of the enamel. (4) To test the hypothesis that the specific sets of laser irradiation conditions previously shown to most effectively interact with enamel to induce resistance to dental caries can be modified for caries inhibition in dentin. The proposed studies will provide information that will lead to an improved method for treating pit and fissure caries, the caries form that is most prevalent today. Further, the treatment of dentinal and root caries may be possible by the use of lasers. Additionally, the studies are expected to lead to the identification of laser irradiation conditions that can be used not only for caries prevention, but also for removal of carious lesions. The development of economical, physically manageable, low energy lasers for caries prevention and removal in the dental office, that operate at specifically designed wavelength, pulse duration, and fluence conditions, is a real and exciting possibility for the future. [unreadable] [unreadable]